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Chapter 4 Lecture
Concepts of Genetics
Tenth Edition
Extensions of
Mendelian Genetics
• Mendel’s work at start of 20th century well
known
• Other biologists verify with crosses of
other organisms
• Exceptions were observed
• Extensions to Mendel’s principles of
heredity
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Cuenot’s Mice
• Cuenot worked with coat color in mice
which come in many colors:
– Black, gray, brown, white, yellow
– But no true breeding yellow mice ever
obtained
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Genes and Alleles Interact to Produce Phenotypes
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Existing alleles subject to mutation
Give rise to new alleles
Single gene can have many alleles
A single gene many phenotypic effects
Single character may be controlled by
many genes
• Expression of gene is affected by
interactions with other genes and with
environment
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• Alternative forms of a gene are called alleles
• Mutation is the source of alleles
• The wild-type allele is the one that occurs
most frequently in nature
• The other alleles of that gene are mutant
• Genes with more than one allele are
polymorphic
– New phenotypes result from changes in
functional activity of gene product
• Eliminating enzyme function
• Changing relative enzyme efficiency
• Changing overall enzyme function
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Multiple alleles for coat color in rabbits
• C gene for coat color
• 4 alleles
• C > cchd > ch > c
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Dominance not always complete
• Many genes have alleles that are neither
dominant nor recessive to one another
• Heterozygotes have an intermediate
phenotype
• Incomplete Dominance
• Phenotype ratio = Genotype ratio
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Incomplete Dominance
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© 2012 Pearson Education, Inc.
Figure 4.1
Codominance
• Two alleles of a gene both produce their
phenotypes when present in a
heterozygote
• Ex: ABO blood group in humans
• Gene I encodes an enzyme involved in
attaching sugars to the surface or RBC
• Three alleles of the I gene: IA IB and i (IO )
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Genes interact when they are expressed
• Many traits characterized by a distinct
phenotype are affected by more than one
gene
• In gene interaction, the cellular function of
numerous gene products contributes to
the development of a common phenotype
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• The term epistasis describes the situation in which
a gene can mask or modify the phenotypic effects of
another gene
• Epistatic interactions often arise because two (or
more) different proteins participate in a common
cellular function
– For example, an enzymatic pathway
Colorless
precursor
Enzyme C
The recessive c allele
encodes an inactive
enzyme
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Colorless
intermediate
Enzyme P
Purple
pigment
The recessive p allele
encodes an inactive
enzyme
• If an individual is homozygous for either
recessive allele, it will not make any
functional enzyme C or enzyme P
– Therefore, in the previous example, the
flowers remain white
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© 2012 Pearson Education, Inc.
Labrador retrievers
Allele B (black) is dominant to b (brown)
Allele E (pigment deposition in hair) is dominant to
e (no deposition so hair is yellow)
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Labrador Retrievers
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The Expression of a Genotype May Be
Influenced by Environmental Effects
• Temperature-sensitive allele:
– An allele whose product is functional only at a
certain temperature.
Two parameters describe the effects of
genes and environment on phenotype
1) Penetrance – proportion of individuals in
a group with a given genotype that actually
show the expected phenotype
– Ex: BRCA1 mutant allele, some people do
not develop breast cancer
– The mutation is said to be incompletely
penetrant
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2) Expressivity – degree or range to which
a genotype is expressed in an individual
– Ex: BRCA1 mutant allele. A woman with the
mutant allele may develop both breast and
ovarian cancer as part of the phenotype, but
another woman with same mutation may only
get breast cancer.
– The mutation is said to have variable
expressivity
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Genes are carried on chromosomes
• A gene is a sequence of DNA that resides
at a particular site on a chromosome
called a locus
• Genes that reside on a single
chromosome are genetically linked
together and this affects pattern of
inheritance
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• X,Y system used for sex
determination by many
animal and plant species
• X is a large chromosome
and encodes many genes
• Y is a small chromosome
with few genes (not
homologous to X in the
traditional sense but has
pairing region for synapsis)
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Thomas Hunt Morgan and the Fly Room
• Early 20th century, Columbia University
• Fruit fly – Drosophila melanogaster
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Figure 4.12
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Figure 4.13
• Thomas Hunt Morgan (1910) showed the Xlinked inheritance in his studies of the white
eyed mutation in Drosophila
• The inheritance pattern was clearly related to
the sex of the parent carrying the mutant
allele
• Reciprocal crosses between white eyed and
red eyed flies did not yield identical results
• White locus is present on the X chromosome
rather than one of the autosomes
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Interesting video to watch on Morgan’s flies
• Calvin Bridges:
http://library.cshl.edu/exhibits/bridges/_pag
es/page1_about.html
• The Fly Room movie:
http://www.theflyroom.com
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• Genes present on the X chromosome
exhibit unique patterns of inheritance due
to the presence of only one X
chromosome in males and two in females
• Males cannot be either homozygous or
heterozygous for X linked genes – they
have only one copy of a gene in a diploid
cell - hemizygous
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• Many traits controlled by X chromosomelinked traits
– Red/green color blindness
– Hemophilia
– Only females are carriers of recessive alleles
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© 2012 Pearson Education, Inc.
Figure 4.14
• Lethal X-linked recessive disorders are
observed most often in males
• Only females can be heterozygous
carriers that do not develop the disorders
• For example: Duchenne muscular
dystrophy
– Muscles slowly degenerate leading to
paralysis
– Onset prior to the age of 6 and lethal around
30
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Concept Check
Hemophilia (reduced blood clotting) is an X-linked
recessive disease in humans. A woman with
hemophilia mates with a man who exhibits normal
blood clotting. What is the probability that their
child will have hemophilia?
Concept Check
Hemophilia (reduced blood clotting) is an X-linked
recessive disease in humans. A woman with
hemophilia mates with a man who exhibits normal
blood clotting. What is the probability that their
child will have hemophilia?
1/2
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.